1. Field of the Invention
The invention relates to the assembly or splicing between two optical components, particularly an optical fibre and another optical component, whereby the latter can e.g. be another optical fibre (particularly a multicore fibre) or a lens or microlens (particularly a graded-index or GRIN lens). The optical fibre can also be a multicore fibre. Another example of an optical element which can be assembled or spliced with the aid of the invention is a lens-prism assembly. The invention also relates to the assembly of an optical component and a substrate, e.g. a semiconductor or metal substrate. The component can e.g. be an optical fibre or a lens.
2. Discussion of the Background
The article by K. Kinoshita entitled xe2x80x9cEnd preparation and fusion splicing of an optical fiber array with CO2 laserxe2x80x9d published in Applied Optics, vol. 18, No. 19, pp 3256-3260, 1979 describes the fusion of optical fibres with the aid of a CO2 laser. The article by K. Egashira entitled xe2x80x9cAnalysis of thermal conditions in CO2 laser splicing of optical fibersxe2x80x9d published in Applied Optics, vol. 16, No. 10, pp 2743-2746, 1977 also relates to fibre-fibre splicing using a CO2 laser.
The article by K. Nakatate et al entitled xe2x80x9cSilica based rod lens for the medical fiberscopexe2x80x9d published in Proceedings SPIE, 1994 relates to a fibre-lens bonding. It uses a technology identical to that of fibre-fibre fusion using an electric arc. It involves a meeting of the surfaces to be contacted.
This procedure is only applicable to small diameter optical elements (less than 200 xcexcm). Microlenses are produced using the same procedures as those used for producing the optical fibres. Thus, the glasses obtained for the lenses melt at temperatures comparable with the fibre fusion temperatures, which obviously makes fusion or welding easier. Consequently, this procedure cannot generally be applied to the production of an assembly of two random optical components.
In general terms, all these procedures lead to a deformation of the contacting surfaces by heating. In the case of fibre-fibre splicing, fusion leads to a deformation of the end of the fused together fibres. Moreover, in general, the optical fibres are prepared beforehand by cleaving, but the perpendicularity of the interface with respect to the fibre axis is not guaranteed in this procedure. Thus, fusion requires a plastic deformation obtained by exerting an axial pressure.
A known method for assembling two random optical components, e.g. a lens and a prism, involves bonding. However, bonding is chemically sensitive to certain solvents and leads to a poor mechanical strength for small surfaces. It also requires the introduction of a material (the adhesive), which reduces the optical quality of the path which can be followed by a beam. Bonding is more particularly used for the assembly of a lens and a multicore fibre with a view to the preparation of microendoscopes.
Endoscopy and in particular microendoscopy enables a medical practitioner to acquire information or images of internal parts of the human body, such as the stomach, lungs, heart, blood vessels or eye.
An apparatus for performing such a procedure is diagrammatically shown in FIG. 1, where reference 2 designates a light source focussed by a lens 4 at the entrance end of a light guide 6. The latter is usually connected to a plurality of optical fibres 8, 10 located on the periphery of a multicore fibre 12. Thus, an illuminating beam 14 can be directed onto an area 16 of an organ to be observed, which reflects radiation 18 onto a lens 20 connected to the entrance end of a multicore fibre 12. As the latter has a coherent bundle of individual cores, the latter consequently transmit the light in an ordered manner between them and the image obtained at the exit end 22 of the multicore fibre corresponds to the image formed at its entrance end. Means for storing, analyzing and/or representing the image can also be provided in combination with this apparatus.
This imaging procedure is e.g. described in the articles by A. Katzir: xe2x80x9cOptical fibers in medicinexe2x80x9d, Scientific American, vol. 260 (5), pp 120-125, 1989 and xe2x80x9cOptical fiber techniques (medicine)xe2x80x9d, Encyclopedia of Physical Science and Technology, vol. 9, pp 630-646, 1987.
FIG. 2 illustrates the presentday production of the lens-multicore fibre assembly. A metal tube 24 maintains the lens 20 in front of the multicore fibre 12 and an adhesive 26 ensures the optical continuity and prevents the lens from passing out of the tube 24. This procedure gives good results, but has the disadvantage of requiring difficult manipulation, of reducing the optical quality by introducing a supplementary medium 26 between the lens and the multicore fibre and of making the endoscope very vulnerable to the necessary heating-based sterilization stages. Moreover, bonding takes place blind in the tube 24 and without any accurate control. In view of the tolerances of the tube, the bonding action is random and very variable.
In general terms, the assembly or splicing of two optical components or an optical component and a substrate by bonding also suffers from a certain fragility and is not compatible with high or very high temperature uses, particularly when a sterilization is necessary.
U.S. Pat. No. 5,208,885 describes a process for producing a connection between a waveguide on a substrate and an optical fibre. A glass paste, whose melting point is lower than the temperature to which the waveguide can be heated is applied to the optical fibre and/or to the waveguide. The glass paste is heated in order to bring about the connection between the fibre and the waveguide.
More specifically, the glass can consist of a borosilicate-aluminium-lead mixture and heating can be brought about using a laser, e.g. a CO2 laser or an excimer laser.
The procedure described in this document does not solve the optical problems, i.e. the optical deterioration and disturbance to the beam when the latter has to traverse the glass connection. The implementation of this procedure with a view to producing an imaging device, e.g. an endoscope is consequently impossible. Moreover, the application given relates to a weld between materials (made from glass) having similar compositions (SiO2/Si substrate with a weakly doped SiO2 fibre) melting at high temperatures, which provides the choice for the weld of multiple glass compositions melting at lower temperatures, as well as different production procedures.
The material adopted in said document for the weld (glass paste) is difficult to dose due to the evaporation of the binder, which considerably modifies the volume thereof.
The paste can also undergo chemical deteriorations making it inappropriate for use in optics. Moreover, the homogenization necessary for reducing diffusion involves a temperature rise up to 1000xc2x0 C., which is unacceptable when certain optical components have to be contacted or welded to one another.
Finally, the use of glasses melting at low temperatures is not necessarily an advantage if their optical properties (refractive index) and thermal properties (expansion) are too different from those of the optical elements. For example, the respective expansion coefficients of a multicore fibre and a lens are respectively 5.10xe2x88x927 and 100.10xe2x88x927.
Finally, for the implementation described in said document (plunging the end of a fibre in a glass bath) does not make it possible to carry out a precise check on the deposited glass quantity, or on the alignment of the elements to be welded prior to the melting of the layer. This procedure also involves a good wettability.
EP-678 486 (Gould Electronics) describes a process for producing a bond or a lateral coating or covering between glass-based components.
The assembly is obtained with the aid of a glass-based composition, which is heated, e.g. with the aid of a CO2 laser or an electric arc. The wettability properties of the surfaces are essential to the assembly.
Here again, the document does not refer to the question of the optical transmission of the materials used. Deterioration to optical properties can occur, e.g. due to:
the stressing of a fibre, which can lead to refractive index variations, thereby disturbing the propagation of the signal,
optical absorption by lead glasses, which can produce an attenuation or colouring of the signal, particularly under the influence of X-rays (e.g. in the case of use of an endoscope),
the presence of residues of the binder in the case of glass pastes, or inhomogeneities, which can produce diffusion.
Thus, the connection described in this document is inappropriate for producing an imaging device, particularly an endoscope. In addition, there again, the application provided relates to the joining of elements which are similar to one another with regards to their compositions and which melt at high temperatures, which offers the choice for numerous glasses melting at low temperatures. Finally, the procedure adopted in this document does not make it possible to precisely check the deposited glass quantity or the alignment of the elements to be welded.
In addition, none of the procedures described hereinbefore is appropriate for producing assemblies of very different materials, e.g. an optical component and a shape memory, plastic, semiconductor or metal substrate. However, a lens-metal tube assembly is used, e.g. in rigid endoscopes.
Another example of such an assembly is that of a shape memory material and an optical component, e.g. an optical fibre.
The only known method making it possible to produce such assemblies is that of bonding, which suffers from the disadvantages referred to hereinbefore (lack of stability of certain solvents, poor mechanical strengths for small surfaces, introduction of a material (the adhesive) which disturbs the optical beams or reduces the optical quality of the path to be followed by a beam). It is therefore desirable to find an assembly procedure making it possible to reduce the optical disturbance between the two elements to be assembled.
The known methods also do not permit the effecting of a precise adjustment of the surfaces to be contacted.
Finally, in the case of a metal-glass weld, conventional welding methods do not make it possible to eliminate deformations of the glass.
The first object of the invention is a process for assembling an optical component and a substrate, which makes it possible to avoid the disadvantages described hereinbefore.
The invention firstly relates to a process for the assembly of an optical component with a substrate comprising:
a first stage of depositing a glass layer on at least one of the two faces or surfaces to be contacted,
a second stage of contacting the two faces or surfaces,
a third stage of heating the glass, leading to a weld between the optical component and the substrate.
The invention also relates to a process for the assembly of two optical components making it possible to avoid the disadvantages referred to hereinafter and which can in particular be applied to the implementation of imaging devices, e.g. endoscopes.
The invention therefore also relates to a process for the assembly of a first and a second optical components comprising:
a first stage of depositing a glass layer on one of the two faces or surfaces to be contacted,
a second stage of contacting the two faces or surfaces,
a third stage of heating the glass leading to a weld between the two optical components.
In both cases, the weld or brazed joint obtained has a high mechanical strength and a good thermal behaviour.
In addition, in both cases, the glass layer is deposited on the active face or faces of the optical component or components, i.e. on the face or faces to be traversed by radiation.
The glass layer is also deposited in situ, without requiring subsequent spreading during assembly. The wettability of the surfaces to be contacted is therefore not essential to the process according to the invention.
The use of a thin glass layer (layer with a thickness between 0.1 and 10 xcexcm) for the weld makes it possible to reduce or avoid parallelism problems between the faces or surfaces to be contacted and avoids deformation of said faces or surfaces. A thin glass layer also does not reduce the optical quality of the components, unlike in the case of an adhesive layer or a drop of glass paste. In particular, the thin layer does not or only slightly disturbs an optical beam traversing it, which is the case when it is e.g. located at the junction of two optical fibres.
Such a glass layer can be used for bonding or welding very different materials. In particular, the process according to the invention is particularly readily applicable to the production of assemblies of optical components, or an optical component and a substrate, having different or very different thermal expansion coefficients. For the glass of the thin layer a choice will then be made of a composition having an expansion coefficient intermediate between those of the components.
In the case of an optical component-substrate bond, the latter can be metallic, plastic, semiconductor or shape memory or can be a metal layer deposited on a shape memory substrate, the optical component then e.g. being a single core optical fibre or a multicore optical fibre. Finally, the substrate may or may not be flat. Thus, it is possible to produce a metal tube-lens assembly of the type used in conventional endoscopy. The invention also relates to an endoscope implementing such an assembly.
In the case of two optical components, one of the two components can be an optical fibre having a single core or can be a multicore fibre, the second optical component being e.g. an optical fibre (once again with a single core or multicore) or a microlens (e.g. a GRIN lens). In addition, the first and/or second optical components can in each case be a lens or a prism.
Moreover, the process according to the invention makes it possible to weld optical components having a random size or diameter, below or above 200 xcexcm.
Another advantage of the glass layer is that as a result of its ductility it absorbs part of the mechanical tensions linked with possible respective expansions of the components. Thus, the final assembly does not suffer from mechanical stresses, which can be encountered in components directly assembled by laser fusion.
Thus, as a result of the glass layer, it is possible to bring about a precise adjustment of the surfaces to be contacted.
In the case of two optical components, an adjustment of the positioning of one element relative to the other can take place prior to heating through the use of the thin layer. The use of a droplet of adhesive, as in U.S. Pat. No. 5208885, does not make it possible to carry out an adjustment prior to the liquefaction of the intermediate glass.
The adjustment of the positioning can also take place during heating with the aid of optical control means.
In particular, when one of the optical components is a multicore fibre, it is possible to carry out a control or check by an interferometric device. The image of the fringes is transmitted by the multicore fibre.
It should be noted that the glass layer is produced on the optically active part of the system. Preferably it does not contain lead, which can oxidize.
The use of a thin glass layer is also of interest for the following reason. A thin layer has a lower glass transition point than the melting point of the same material in the form of a macroscopic volume. Thus, there is a low temperature melt, which is advantageous with respect to the optical component or components to be assembled, which could be deteriorated by an excessive temperature rise.
The thinness of the layer makes it possible to separate the welding properties from the melting properties. A progressive softening of the layer takes place during heating, which permits an adjustment of the surfaces. The temperature is increased in order to make the actual weld, which is linked with the activation energies at the interfaces between the elements present. Thus, preferably a two-stage heating is used. A first stage makes it possible to reach the softening point of the thin layer. In a second stage the temperature is raised to an adequate value to make the actual weld. Said second stage can be brief (e.g. a few minutes or less).
The heating stage can e.g. take place by electric arc or filament (operating by the Joule effect) or by laser. Laser heating is better temperature-controlled. In the case of a laser and for an optical fibre-optical component or optical fibre-substrate assembly, it can be of interest to arrange the laser beam and fibre-component or fibre-substrate assembly in such a way that, at the laser beam impact point, the said beam is displaced to the side of the optical fibre. In this way the absorption and conductivity of the fibre are combined. Thus, the heated volume is displaced to the side of the latter and the heating either does not affect or only slightly affects the substrate or component. As a function of the nature of the latter, a displacement by a distance, measured between the centre of the beam and the end of the fibre, between a few and a few hundred micrometres (e.g. between 50 and 200 xcexcm or 300 xcexcm, or between 90 and 170 xcexcm) can be appropriate.
Compared with other methods (electric arc, filaments), laser heating has the advantage of offering a considerable adaptability. The focussing and size of the beam are adaptable to the type of surface or object to be welded.
It is possible to preheat the thin layer in order to increase its adhesion without deforming it. This preheating stage can be performed prior to the contacting of the substrate and the optical component or element, or the optical components or elements with one another. It makes it possible to reinforce the bond (combination of covalent and ionic bonds) between the glass layer and the surface on which it is deposited, which then aids the actual welding.
Preferably, the glass used is an evaporatable glass which, during its evaporation, retains the same chemical composition and same physical properties as the original material.
For the glass layer it is possible to choose a glass with a glass transition point between 400 and 600xc2x0 C., or between 400 and 500xc2x0 C., e.g. a glass incorporating a silica matrix doped with sodium and boron, e.g. also a silica matrix doped with a B2O3-Al2O3Na2O-K2O mixture. Germanium-doped glasses also have a low glass transition point. Thus, the glass transition takes place at relatively low temperatures compared with the critical temperatures of most optical components, e.g. optical fibres having one or more cores. In the case of optical components having a certain fragility with respect to thermal shocks or a thermal deformation risk of the index profile (e.g. GRIN lens used in endoscopy), this point can be important.
The invention also relates to an assembly of an optical component and a substrate having, apart from the component and the substrate, a glass layer located at the component-substrate interface.
The invention also relates to an assembly of two optical components having, apart from the two components, a thin glass layer located at the interface between these two components.
As has been explained hereinbefore, the substrate can be metallic, semiconductor, plastic or shape memory type and the optical component or components can be optical fibre or fibres having a single or several cores, or can be a microlens (GRIN) or a lens or even a prism. The glass can be an evaporatable glass, as defined hereinbefore. The invention also relates to an endoscope having a multicore fibre and a lens fixed to the end of the fibre by means of a glass layer located at the lens-fibre interface.
The invention also relates to an endoscope having a multicore fibre, a lens connected to said multicore fibre, means for illuminating an area to be observed, the connection between the lens and the fibre being constituted by a material able to withstand the sterilization temperature and the humid heat of an autoclave.
The lens-fibre connection is e.g. brought about by a thin glass layer according to one or other of the embodiment described hereinbefore.